The invention relates to silicon/graphite/carbon composites (Si/G/C composites), to a method for producing them, and to their use as electrode active material in lithium ion batteries.
Rechargeable lithium ion batteries are presently the commercialized electrochemical energy storage devices with the highest energy densities, of up to 250 Wh/kg. They are utilized especially in the sector of portable electronics, for tools, and also for means of transport, such as bicycles or automobiles, for example. Especially for application in automobiles, however, it is necessary to achieve further significant increase in the energy density of the batteries, in order to obtain longer ranges for the vehicles.
Used in particular as negative electrode material (“anode”) is graphitic carbon. Features of the graphitic carbon are its stable cycling properties and its decidedly high handling safety, in comparison to lithium metal which is used in lithium primary cells. A key argument in favor of the use of graphitic carbon in negative electrode materials lies in the small volume changes of the host material that are associated with the intercalation and deintercalation of lithium, i.e., the electrode remains approximately stable. For instance, for the intercalation of lithium in graphitic carbon, a volume increase of only about 10% is measured for the limiting stoichiometry of LiC6. A disadvantage, however, is its relatively low electrochemical capacity of in theory 372 mAh/g, which is only about one tenth of the electrochemical capacity theoretically achievable with lithium metal.
In anodes for lithium ion batteries where the electrode active material is based on silicon (as the material with the highest known storage capacity for lithium ions; 4199 mAh/g), the silicon may experience an extreme change in volume of up to about 300% on charging and/or discharging with lithium. As a result of this change in volume there is a severe mechanical stress on the active material and on the electrode structure as a whole, leading, through electrochemical milling, to a loss of electrical contacting and hence to the destruction of the electrode, with accompanying loss of capacity. Moreover, the surface of the silicon anode material used reacts with constituents of the electrolyte, accompanied by continuous formation of passivating protective layers (solid electrolyte interface; SEI), leading to an irreversible loss of mobile lithium.
In order to solve the problem of the severe volume expansion of the active material and of the formation of SEI in Si-containing anodes, the last decade has seen a variety of approaches toward electrochemical stabilization of Si-containing electrode active materials (an overview is given by A. J. Appleby et al., J. Power Sources 2007, 163, 1003-1039).
One possible solution is to use the silicon-containing active material not in pure form, but instead in combination with carbon. In this case it is possible on the one hand to insert the Si-containing active material in the form of a physical mixture with graphite into the electrode coating (cf. EP 1730800 B1), or to combine the two elements, silicon and carbon, structurally in the form of a composite material (an overview is given by M. Rossi et al., J. Power Sources 2014, 246, 167-177).
Graphite and structurally related carbons are relatively soft, have very good electrical conductivity, possess a low mass, and feature a low change in volume when charging/discharging. For these reasons, carbon-based anodes, as is known, have a very good electrochemical stability of several hundred cycles. By combination of the advantages of the two elements (silicon (Si) with high capacity, graphite (G) and/or amorphous carbon (C) with high stability), electrode active materials based on Si/C or Si/G/C composites possess a more stable cycling behavior than the pure silicon, with a capacity increased by comparison with that of pure graphite.
Composites of this kind can be produced, according to EP 1363341 A2, by chemical vapor deposition of carbon on silicon.
Also known is the production of Si/C composites by reactive milling of silicon with carbon or carbon precursors, and subsequent carbonization; see, for example, US 20040137327 A1.
The embedding of silicon particles into an organic C precursor matrix with subsequent carbonization also leads to Si/C or Si/G/C composites; see, for example, US 20050136330 A1. C precursors contemplated here are primarily hydrocarbons, carbohydrates, and a multiplicity of polymers, leading, according to their composition and structure, to graphitizable (soft) or nongraphitizable (hard) carbons.
A distinction is made below between composites which comprise only nanoscale silicon, embedded in an amorphous carbon matrix (Si/C composites), and materials which additionally include one or more crystalline graphite cores within an Si/C shell (Si/G/C composites). In a variety of disclosures, the graphite content in particular has had beneficial consequences for the conductivity and structural stability; cf., e.g., EP 2573845 A1.
Another distinction made in this invention is between whether nanoscale silicon is present in the surrounding C matrix in the form of “aggregates” (i.e., nanoscale primary particles are intergrown firmly with one another, via sinter necks, for example, and can no longer be separated from one another), or in the form of nonaggregated, isolated individual particles, which may optionally form loose particle assemblies (“agglomerates”).
Si/C composites containing aggregated Si nanoparticles are described for example in WO 2013031993 A1, where production takes place via a C coating of an aggregated Si starting material.
Also known are Si/C composites with nonaggregated silicon. The use of a nonaggregated nanoscale silicon powder in the polycondensation of mono- and/or polyhydroxyaromatic compounds with an aldehyde, in the presence of a catalyst, and subsequent carbonization for the production of nanostructured Si/C composites, is described in WO 2010006763 A1.
CA 2752844 A1 discloses a method for the C coating of Si and SiOx particles, where the composite particles obtained include a high fraction of at least 50% of the embedded nanoscale Si particles in nonaggregated or nonsintered form. A disadvantage of the Si/C composites specified here is that they contain no graphite for improving the conductivity and structural stability. Moreover, the C contents described are very low (<30%), and so, while the C coating does improve the conductivity of the silicon surface, it possesses no stabilizing effect in relation to the volume expansion of the silicon particles.
Si/G/C composites as claimed in US 20130302675 A1 include a (porous) graphite core and Si particles aggregated on the surface, these particles having a coating of amorphous carbon. One possible drawback of this composite is that it has only a little amorphous carbon (1-10%) between the Si particles, thus ruling out the possibility of ensuring adequate stability by buffering the change in volume of the silicon. Moreover, the composites described in US 20130302675 A1 contain only very low silicon levels of approximately 5% and hence contain electrochemical capacities which are only just above those of graphite (˜400 mAh/g) and are not relevant for the majority of target applications.
CN 101210112 A describes how the aggregation of the Si particles is reduced by embedment into an organic polymer coating on a graphite core (but the coating there is not carbonized to inorganic carbon, harboring possible drawbacks in respect of conductivity and mechanical strength). It has been found that these structures are disadvantageous in terms of conductivity and mechanical strength.